<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>Default semantic properties for Custom Elements</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li             { font-weight: bold;   }
	.toc > li li          { font-weight: normal; }
	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @supports not (display:grid) { */
		.toc > li             { margin: 1.5rem 0;    }
		.toc > li li          { margin: 0.3rem 0;    }
		.toc > li li li       { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}

		.toc li {
			clear: both;
		}

		:not(li) > .toc              { margin-left:  5rem; }
		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc              { margin-left:  4rem; }
			.toc .secno                  { margin-left: -4rem; }
			.toc > li li li              { margin-left:  1rem; }
			.toc > li li li .secno       { margin-left: -5rem; }
			.toc > li li li li .secno    { margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good table positioning:
		   "content column" is 50ems wide at max; less on smaller screens.
		   Extra space (after ToC + content) is empty on the right.

		   1. When table < content column, centers table in column.
		   2. When content < table < available, left-aligns.
		   3. When table > available, fills available + scroll bar.
		*/ 
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
  <link href="https://www.w3.org/StyleSheets/TR/2016/W3C-UD" rel="stylesheet">
  <link href="https://wicg.github.io/aom" rel="canonical">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
<style>/* style-dfn-panel */

.dfn-panel {
    position: absolute;
    z-index: 35;
    height: auto;
    width: -webkit-fit-content;
    width: fit-content;
    max-width: 300px;
    max-height: 500px;
    overflow: auto;
    padding: 0.5em 0.75em;
    font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
    background: #DDDDDD;
    color: black;
    border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0; }
.dfn-panel li { list-style: inside; }
.dfn-panel.activated {
    display: inline-block;
    position: fixed;
    left: .5em;
    bottom: 2em;
    margin: 0 auto;
    max-width: calc(100vw - 1.5em - .4em - .5em);
    max-height: 30vh;
}

.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-syntax-highlighting */
pre.idl.highlight { color: #708090; }
.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">Default semantic properties for Custom Elements</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Unofficial Proposal Draft, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wicg.github.io/aom">https://wicg.github.io/aom</a>
     <dt class="editor">Editors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:aboxhall@google.com">Alice Boxhall</a> (<span class="p-org org">Google</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jcraig@apple.com">James Craig</a> (<span class="p-org org">Apple</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dmazzoni@google.com">Dominic Mazzoni</a> (<span class="p-org org">Google</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:surkov.alexander@gmail.com">Alexander Surkov</a> (<span class="p-org org">Mozilla</span>)
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 the Contributors to the Default semantic properties for Custom Elements Specification, published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a> under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a>.
A human-readable <a href="http://www.w3.org/community/about/agreements/cla-deed/">summary</a> is available. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This specification describes the additions to existing

specifications which will make it possible for web authors
to programmatically express semantics for Web Components.</p>
  </div>
  <div data-fill-with="at-risk"></div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This specification was published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a>.
  It is not a W3C Standard nor is it on the W3C Standards Track.

  Please note that under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a> there is a limited opt-out and other conditions apply.

  Learn more about <a href="http://www.w3.org/community/">W3C Community and Business Groups</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#custom-element-semantics"><span class="secno">1</span> <span class="content">Defining <span>custom element</span> semantics</span></a>
     <ol class="toc">
      <li>
       <a href="#custom-elements-intro"><span class="secno">1.1</span> <span class="content">Introduction</span></a>
       <ol class="toc">
        <li><a href="#custom-element-definition-semantics"><span class="secno">1.1.1</span> <span class="content">Defining custom element semantics as part of the <span>custom element definition</span></span></a>
        <li><a href="#element-semantics"><span class="secno">1.1.2</span> <span class="content">Defining per-instance custom element semantics</span></a>
       </ol>
      <li><a href="#element-definition-options"><span class="secno">1.2</span> <span class="content">Changes to custom element definition</span></a>
     </ol>
    <li>
     <a href="#semantics-precedence"><span class="secno">2</span> <span class="content">ARIA semantic precedence between <code class="idl"><span>ElementDefinitionOptions</span></code>, <code class="idl"><span>ElementInternals</span></code> and ARIA properties</span></a>
     <ol class="toc">
      <li><a href="#semantics-precedence-intro"><span class="secno">2.1</span> <span class="content">Introduction</span></a>
      <li>
       <a href="#custom-elements-semantics-mapping"><span class="secno">2.2</span> <span class="content">Mapping semantics to the accessibility tree</span></a>
       <ol class="toc">
        <li><a href="#custom-eleent-semantics"><span class="secno">2.2.1</span> <span class="content">Rules for exposing semantics of <span>custom elements</span></span></a>
       </ol>
     </ol>
    <li><a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#idl-index"><span class="secno"></span> <span class="content">IDL Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="custom-element-semantics"><span class="secno">1. </span><span class="content">Defining <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element" id="ref-for-custom-element">custom element</a> semantics</span><a class="self-link" href="#custom-element-semantics"></a></h2>
   <section class="non-normative">
    <h3 class="heading settled" data-level="1.1" id="custom-elements-intro"><span class="secno">1.1. </span><span class="content">Introduction</span><a class="self-link" href="#custom-elements-intro"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>Authors may potentially provide semantics for custom elements
in two ways:</p>
    <ul>
     <li data-md>
      <p>as part of the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition">custom element definition</a></p>
     <li data-md>
      <p>via an object provided to each instance of the custom element.</p>
    </ul>
    <h4 class="heading settled" data-level="1.1.1" id="custom-element-definition-semantics"><span class="secno">1.1.1. </span><span class="content">Defining custom element semantics as part of the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition①">custom element definition</a></span><a class="self-link" href="#custom-element-definition-semantics"></a></h4>
    <p>Authors may provide <em>immutable</em> default semantics for a custom element
by setting properties via the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions">ElementDefinitionOptions</a></code> object
passed in to the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define" id="ref-for-dom-customelementregistry-define">CustomElementRegistry.define()</a></code> method.</p>
    <p>The properties set on the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions①">ElementDefinitionOptions</a></code> object
become the default values to be used when <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-map" id="ref-for-ordered-map">mapping</a> the custom element to an <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessible-object" id="ref-for-dfn-accessible-object">accessible object</a>.</p>
    <p class="note" role="note"><span>Note:</span> this is analogous to creating an "immutable class variable" -
these semantic properties are associated with
the custom element definition,
not with each custom element instance.
The semantics they define apply to <em>all</em> instances
of the custom element.</p>
    <aside class="example" id="custom-tab-example">
     <a class="self-link" href="#custom-tab-example"></a> For example, an author creating a custom tab control
may define three custom elements for the individual tabs,
the tab list and the tab panel: 
<pre class="language-js highlight"><c- kr>class</c-> TabListElement <c- kr>extends</c-> HTMLElement <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-tablist"</c-><c- p>,</c-> TabListElement<c- p>,</c->
                      <c- p>{</c-> role<c- o>:</c-> <c- u>"tablist"</c-><c- p>,</c-> ariaOrientation<c- o>:</c-> <c- u>"horizontal"</c-> <c- p>});</c->

<c- kr>class</c-> TabElement <c- kr>extends</c-> HTMLElement <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-tab"</c-><c- p>,</c-> TabElement<c- p>,</c->
                      <c- p>{</c-> role<c- o>:</c-> <c- u>"tab"</c-> <c- p>});</c->

<c- kr>class</c-> TabPanelElement <c- kr>extends</c-> HTMLElement <c- p>{</c-> <c- p>...</c-> <c- p>}</c->
customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-tabpanel"</c-><c- p>,</c-> TabPanelElement<c- p>,</c->
                      <c- p>{</c-> role<c- o>:</c-> <c- u>"tabpanel"</c-> <c- p>});</c->
</pre>
     <p>When a <code>&lt;custom-tab></code> element is <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#mapping_general" id="ref-for-mapping_general">mapped</a> into the <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessibility-tree" id="ref-for-dfn-accessibility-tree">accessibility tree</a>,
by default it will have a <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#mapping_role" id="ref-for-mapping_role">mapped role</a> of <code>tab</code>.</p>
     <p>This is analogous to how a <code>&lt;button></code> element is, by default,
mapped to an <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessible-object" id="ref-for-dfn-accessible-object①">accessible object</a> with a <a data-link-type="dfn" href="https://www.w3.org/TR/html-aam-1.0/#details-id-17" id="ref-for-details-id-17">role of <code>button</code></a>.</p>
    </aside>
    <h4 class="heading settled" data-level="1.1.2" id="element-semantics"><span class="secno">1.1.2. </span><span class="content">Defining per-instance custom element semantics</span><a class="self-link" href="#element-semantics"></a></h4>
    <p class="note" role="note"><span>Note:</span> see <a href="https://github.com/w3c/webcomponents/issues/758">Web Components issue #758</a> for up to date information on <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals">ElementInternals</a></code>.</p>
    <p>A <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element" id="ref-for-custom-element①">custom element</a> author may use the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals①">ElementInternals</a></code> object,
created via <code class="idl"><a data-link-type="idl">createInternals()</a></code>,
to modify the semantic state of an instance of a custom element
in response to user interaction.</p>
    <p>The properties set on the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals②">ElementInternals</a></code> object
are used when <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-map" id="ref-for-ordered-map①">mapping</a> the element
to an <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessible-object" id="ref-for-dfn-accessible-object②">accessible object</a>.</p>
    <p class="note" role="note"><span>Note:</span> this is analogous to setting an "instance variable" -
a copy of these semantic properties is created for each instance
of the custom element.
The semantics defined in each
apply only to their associated custom element instance object.</p>
    <aside class="example" id="custom-tab-example-continued">
     <a class="self-link" href="#custom-tab-example-continued"></a> For example, the author creating the <a href="#custom-tab-example"><code>&lt;custom-tab></code></a> and related elements
may use the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals③">ElementInternals</a></code> object
to modify the semantic details for the elements
as they change in response to user interaction. 
<pre class="language-js highlight"><c- kr>class</c-> CustomTab <c- kr>extends</c-> HTMLElement <c- p>{</c->
  #internals <c- o>=</c-> <c- kc>null</c-><c- p>;</c->
  #tablist <c- o>=</c-> <c- kc>null</c-><c- p>;</c->
  #tabpanel <c- o>=</c-> <c- kc>null</c-><c- p>;</c->

  constructor<c- p>()</c-> <c- p>{</c->
    <c- kr>super</c-><c- p>();</c->
    <c- k>this</c-><c- p>.</c->#internals <c- o>=</c-> customElements<c- p>.</c->createInternals<c- p>(</c-><c- k>this</c-><c- p>);</c->
    <c- k>this</c-><c- p>.</c->#internals<c- p>.</c->role <c- o>=</c-> <c- u>"tab"</c-><c- p>;</c->
  <c- p>}</c->

  <c- c1>// Observe the custom "active" attribute.</c->
  <c- kr>static</c-> get observedAttributes<c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>[</c-><c- u>"active"</c-><c- p>];</c-> <c- p>}</c->

  connectedCallback<c- p>()</c-> <c- p>{</c->
    <c- k>this</c-><c- p>.</c->#tablist <c- o>=</c-> <c- k>this</c-><c- p>.</c->parentElement<c- p>;</c->
  <c- p>}</c->

  setTabPanel<c- p>(</c->tabpanel<c- p>)</c-> <c- p>{</c->
    <c- k>if</c-> <c- p>(</c->tabpanel<c- p>.</c->localName <c- o>!==</c-> <c- u>"custom-tabpanel"</c-> <c- o>||</c-> tabPanel<c- p>.</c->id <c- o>===</c-> <c- u>""</c-><c- p>)</c->
      <c- k>return</c-><c- p>;</c->  <c- c1>// fail silently</c->

    <c- k>this</c-><c- p>.</c->#tabpanel <c- o>=</c-> tabpanel<c- p>;</c->
    tabpanel<c- p>.</c->setTab<c- p>(</c-><c- k>this</c-><c- p>);</c->
    <c- k>this</c-><c- p>.</c->#internals<c- p>.</c->ariaControls <c- o>=</c-> tabPanel<c- p>;</c->    <c- c1>// does not reflect</c->
  <c- p>}</c->

  <c- c1>// ... setters/getters for custom properties which reflect to attributes</c->

  attributeChangedCallback<c- p>(</c->name<c- p>,</c-> oldValue<c- p>,</c-> newValue<c- p>)</c-> <c- p>{</c->
    <c- k>switch</c-><c- p>(</c->name<c- p>)</c-> <c- p>{</c->
      <c- k>case</c-> <c- u>"active"</c-><c- o>:</c->
        <c- a>let</c-> active <c- o>=</c-> <c- p>(</c->newValue <c- o>!=</c-> <c- kc>null</c-><c- p>);</c->
        <c- k>this</c-><c- p>.</c->#tabpanel<c- p>.</c->shown <c- o>=</c-> active<c- p>;</c->

        <c- c1>// When the custom "active" attribute changes,</c->
        <c- c1>// keep the accessible "selected" state in sync.</c->
        <c- k>this</c-><c- p>.</c->#internals<c- p>.</c->ariaSelected <c- o>=</c-> <c- p>(</c->newValue <c- o>!==</c-> <c- kc>null</c-><c- p>);</c->

        <c- k>if</c-> <c- p>(</c->selected<c- p>)</c->
          <c- k>this</c-><c- p>.</c->#tablist<c- p>.</c->setSelectedTab<c- p>(</c-><c- k>this</c-><c- p>);</c->  <c- c1>// ensure no other tab has "active" set</c->
        <c- k>break</c-><c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->
<c- p>}</c->

customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-tab"</c-><c- p>,</c-> CustomTab<c- p>,</c-> <c- p>{</c-> role<c- o>:</c-> <c- u>"tab"</c-><c- p>,</c-> needsElementInternals<c- o>:</c-> <c- kc>true</c-> <c- p>});</c->
</pre>
     <p>Authors using these elements may override the default semantics
using ARIA attributes as normal -
see <a href="#semantics-precedence">§ 2 ARIA semantic precedence between ElementDefinitionOptions, ElementInternals and ARIA properties</a>.</p>
     <p>For example, an author may modify the appearance
of a <code>&lt;custom-tablist></code> element to appear as a vertical list.
They could add an <code>aria-orientation</code> attribute to indicate this,
overriding the default semantics set in the custom element definition.</p>
<pre class="language-html highlight"><c- p>&lt;</c-><c- f>custom-tablist</c-> <c- e>aria-orientation</c-><c- o>=</c-><c- s>"vertical"</c-> <c- e>class</c-><c- o>=</c-><c- s>"vertical-tablist"</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-> <c- e>selected</c-><c- p>></c->Tab 1<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-><c- p>></c->Tab 2<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-><c- p>></c->Tab 3<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
<c- p>&lt;/</c-><c- f>div</c-><c- p>></c->
</pre>
     <p>Because the author-provided role overrides the default role,
the <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#mapping_general" id="ref-for-mapping_general①">mapped</a> role will be based on the author-provided role in each case.</p>
    </aside>
   </section>
   <h3 class="heading settled" data-level="1.2" id="element-definition-options"><span class="secno">1.2. </span><span class="content">Changes to custom element definition</span><a class="self-link" href="#element-definition-options"></a></h3>
   <p><strong class="advisement"> This section represents changes which should be made to <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-core-concepts">HTML 5 §4.13.3 Core concepts</a>, <span spec-section="#custom-element-definition"></span>, <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-api">HTML 5 §4.13.4 The CustomElementRegistry interface</a>, <span spec-section="#element-definition"></span>, <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#upgrades">HTML 5 §4.13.5 Upgrades</a> and <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions">HTML 5 §4.13.6 Custom element reactions</a>.</strong></p>
   <p><em>[...]</em></p>
   <p>A custom element may have semantics defined when the custom element is <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition②">defined</a>. Otherwise, an autonomous custom element does not have any special meaning: it represents its children. A customized built-in element inherits the semantics of the element that it extends.</p>
   <p><em>[...]</em></p>
   <p>A custom element definition [includes]:</p>
   <dl>
    <dt>A set of default values for <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="semantic-properties">semantic properties</dfn> (optional)
    <dd> A map, whose keys are each an attribute in either the <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole">AccessibilityRole</a></code> or <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes">AriaAttributes</a></code> interface mixin.
    The corresponding values are <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString">DOMString</a></code>. 
   </dl>
   <p><em>[...]</em></p>
<pre class="idl highlight def">// Existing IDL
/*
[Exposed=Window]
interface CustomElementRegistry {
  [CEReactions] void define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions options);
  // ...
};

dictionary ElementDefinitionOptions {
  DOMString extends;
};
*/
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions②"><c- n>ElementDefinitionOptions</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole①"><c- n>AccessibilityRole</c-></a>;
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions③"><c- n>ElementDefinitionOptions</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes①"><c- n>AriaAttributes</c-></a>;

<c- b>dictionary</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="dictionary" data-export id="dictdef-elementinternals"><code><c- g>ElementInternals</c-></code></dfn> {};

<a class="n" data-link-type="idl-name" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals④"><c- n>ElementInternals</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole②"><c- n>AccessibilityRole</c-></a>;
<a class="n" data-link-type="idl-name" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑤"><c- n>ElementInternals</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes②"><c- n>AriaAttributes</c-></a>;
</pre>
   <p><strong>Element definition</strong> is a process of adding a <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition③">custom element definition</a> to the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry" id="ref-for-customelementregistry">CustomElementRegistry</a></code>.
This is accomplished by the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define" id="ref-for-dom-customelementregistry-define①">define()</a></code> method.
When invoked, the <code class="idl"><a data-link-type="idl">define(name, constructor, options)</a></code> method must run these steps:</p>
   <p><em>[...]</em></p>
   <ol start="11">
    <li>
     Run the following substeps: 
     <ol>
      <li>Let <var>semantics</var> be an empty map. 
      <li>
       For each key defined in <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole③">AccessibilityRole</a></code> and <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes③">AriaAttributes</a></code>: 
       <ol>
        <li>If the key exists in <var>options</var>,
            add an entry to <var>semantics</var> with that key
            and the value provided in <var>options</var>. 
       </ol>
      <li>If <var>semantics</var> is empty, set it to <code>null</code>. 
     </ol>
    <li>Let <var>definition</var> be
    a new <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition④">custom element definition</a> with name <var>name</var>,
    local name <var>local name</var>,
    constructor <var>constructor</var>,
    observed attributes <var>observedAttributes</var>,
    lifecycle callbacks <var>lifecycleCallbacks</var>,
    and, if <var>semantics</var> is non-null, <a data-link-type="dfn" href="#semantic-properties" id="ref-for-semantic-properties">semantic properties</a> <var>semantics</var>.
    <li> Add <var>definition</var> to this <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry" id="ref-for-customelementregistry①">CustomElementRegistry</a></code>. 
   </ol>
   <p><em>[...]</em></p>
   <h2 class="heading settled" data-level="2" id="semantics-precedence"><span class="secno">2. </span><span class="content">ARIA semantic precedence between <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions④">ElementDefinitionOptions</a></code>, <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑥">ElementInternals</a></code> and ARIA properties</span><a class="self-link" href="#semantics-precedence"></a></h2>
   <section class="non-normative">
    <h3 class="heading settled" data-level="2.1" id="semantics-precedence-intro"><span class="secno">2.1. </span><span class="content">Introduction</span><a class="self-link" href="#semantics-precedence-intro"></a></h3>
    <p><em>This section is non-normative</em></p>
    <p>In general, the precedence of semantic properties is that
any ARIA property set directly on the <code class="idl"><a data-link-type="idl" href="https://dom.spec.whatwg.org/#element" id="ref-for-element">Element</a></code> (either via setting an attribute or via the associated <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-reflect" id="ref-for-concept-reflect">reflected</a> property)
overrides a value for the same property on
the <code>Element</code>'s <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑦">ElementInternals</a></code> object,
and any ARIA property set either on the <code>Element</code> or the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑧">ElementInternals</a></code> object
will override a value set via the <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define" id="ref-for-dom-customelementregistry-define②">CustomElementRegistry.define()</a></code> method.</p>
    <aside class="example" id="example-0078eabd">
     <a class="self-link" href="#example-0078eabd"></a> Suppose an author had created a custom checkbox element: 
<pre class="language-js highlight"><c- kr>class</c-> CustomCheckbox <c- kr>extends</c-> HTMLElement <c- p>{</c-> <c- d>/* ... */</c-> <c- p>}</c->
customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-checkbox"</c-><c- p>,</c-> CustomCheckbox<c- p>,</c->
                      <c- p>{</c-> role<c- o>:</c-> <c- u>"checkbox"</c-><c- p>,</c-> ariaChecked<c- o>:</c-> <c- u>"false"</c-> <c- p>});</c->
</pre>
     <p>An author using a <code>&lt;custom-checkbox></code> element
could use the reflected ARIA properties/content attributes to override the
default values, just as they would when using a native element:</p>
     <p></p>
<pre class="highlight"><c- p>&lt;</c-><c- f>--</c-> <c- e>ARIA</c-> <c- e>role</c-> <c- e>overrides</c-> <c- e>implicit</c-> <c- e>role</c-> <c- e>--</c-><c- p>></c->
<c- p>&lt;</c-><c- f>input</c-> <c- e>type</c-><c- o>=</c-><c- s>"checkbox"</c-> <c- e>role</c-><c- o>=</c-><c- s>"radio"</c-><c- p>></c->

<c- p>&lt;</c-><c- f>--</c-> <c- e>ARIA</c-> <c- e>role</c-> <c- e>overrides</c-> <c- e>custom</c-> <c- e>element</c-> <c- e>role</c-> <c- e>--</c-><c- p>></c->
<c- p>&lt;</c-><c- f>custom-checkbox</c-> <c- e>role</c-><c- o>=</c-><c- s>"radio"</c-><c- p>></c->
</pre>
    </aside>
    <aside class="example" id="example-45a2b920">
     <a class="self-link" href="#example-45a2b920"></a> In the implementation of the <a href="#custom-tab-example"><code>&lt;custom-tab></code></a> element,
if the author explicitly defined the default value for <code>ariaSelected</code> in the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition⑤">custom element definition</a>,
then the value set on the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑨">ElementInternals</a></code> object
in the <code>attributeChangedCallback()</code> method
would override that default value: 
<pre class="language-js highlight"><c- kr>class</c-> TabElement <c- kr>extends</c-> HTMLElement <c- p>{</c->
  <c- c1>// ... many details omitted</c->

  <c- c1>// When the custom "active" attribute changes,</c->
  <c- c1>// keep the accessible checked state in sync.</c->
  attributeChangedCallback<c- p>(</c->name<c- p>,</c-> oldValue<c- p>,</c-> newValue<c- p>)</c-> <c- p>{</c->
    <c- k>switch</c-><c- p>(</c->name<c- p>)</c-> <c- p>{</c->
      <c- k>case</c-> <c- u>"active"</c-><c- o>:</c->
        <c- a>let</c-> selected <c- o>=</c-> <c- p>(</c->newValue <c- o>!=</c-> <c- kc>null</c-><c- p>);</c->
        <c- k>this</c-><c- p>.</c->#tabpanel<c- p>.</c->shown <c- o>=</c-> selected<c- p>;</c->

        <c- c1>// Note: overrides value set in custom element definition.</c->
        <c- k>this</c-><c- p>.</c->#internals<c- p>.</c->ariaSelected <c- o>=</c-> selected<c- p>;</c->

        <c- c1>// ensure no other tab has "selected" set</c->
        <c- k>if</c-> <c- p>(</c->selected<c- p>)</c->
          <c- k>this</c-><c- p>.</c->#tablist<c- p>.</c->setSelectedTab<c- p>(</c-><c- k>this</c-><c- p>);</c->
        <c- k>break</c-><c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->
<c- p>}</c->

<c- c1>// ariaSelected value defined here is overridden in attributeChanged callback</c->
customElements<c- p>.</c->define<c- p>(</c-><c- u>"custom-tab"</c-><c- p>,</c-> CustomTab<c- p>,</c->
                      <c- p>{</c-> role<c- o>:</c-> <c- u>"tab"</c-><c- p>,</c-> ariaSelected<c- o>:</c-> <c- u>"false"</c-> <c- p>});</c->
</pre>
     <p>Then authors using the <code>&lt;custom-tablist></code> and <code>&lt;custom-tab></code> elements
as a listbox could still override any semantic properties,
including those set on the <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals①⓪">ElementInternals</a></code> object,
but no semantic implementation details would leak out:</p>
<pre class="language-html highlight"><c- p>&lt;</c-><c- f>custom-tablist</c-> <c- e>aria-orientation</c-><c- o>=</c-><c- s>"vertical"</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>--</c-> <c- e>You</c-> <c- e>would</c-> <c- e>probably</c-> <c- e>never</c-> <c- e>want</c-> <c- e>to</c-> <c- e>do</c-> <c- e>this</c->, <c- e>but</c-> <c- e>it</c-> <c- e>would</c-> <c- e>work</c-> <c- e>--</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-> <c- e>selected</c-> <c- e>aria-selected</c-><c- o>=</c-><c- s>"false"</c-><c- p>></c->Option 1<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-><c- p>></c->Option 2<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>custom-tab</c-><c- p>></c->Option 3<c- p>&lt;/</c-><c- f>custom-tab</c-><c- p>></c->
<c- p>&lt;/</c-><c- f>custom-tablist</c-><c- p>></c->
</pre>
    </aside>
   </section>
   <h3 class="heading settled" data-level="2.2" id="custom-elements-semantics-mapping"><span class="secno">2.2. </span><span class="content">Mapping semantics to the accessibility tree</span><a class="self-link" href="#custom-elements-semantics-mapping"></a></h3>
   <p><strong class="advisement"> This section represents changes which should be made to <a href="https://www.w3.org/TR/html-aam-1.0/#mapping-html-to-accessibility-apis">HTML Accessibility API Mappings 1.0 §mapping-html-to-accessibility-apis</a>.</strong></p>
   <h4 class="heading settled" data-level="2.2.1" id="custom-eleent-semantics"><span class="secno">2.2.1. </span><span class="content">Rules for exposing semantics of <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element" id="ref-for-custom-element②">custom elements</a></span><a class="self-link" href="#custom-eleent-semantics"></a></h4>
   <p>If an element is <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-element-custom" id="ref-for-concept-element-custom">custom</a>,
for each key in the element’s <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals①①">ElementInternals</a></code> object,
the associated value must be mapped to accessibility APIs
as if they were a property set on the element.</p>
   <p>If the element’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition" id="ref-for-custom-element-definition⑥">custom element definition</a> includes <var>semantics</var>,
the value of each property in <var>semantics</var> must be <a data-link-type="dfn" href="https://www.w3.org/TR/core-aam-1.1/#mapping_general" id="ref-for-mapping_general②">mapped</a> to accessibility APIs as if they were set on the element,
unless there is a conflicting value for the same property
in the element’s <code class="idl"><a data-link-type="idl" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals①②">ElementInternals</a></code> object,
or in a property set on the element.</p>
  </main>
  <div data-fill-with="conformance">
   <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
   <p> Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology.
            The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
            in the normative parts of this document
            are to be interpreted as described in RFC 2119.
            However, for readability,
            these words do not appear in all uppercase letters in this specification. </p>
   <p> All of the text of this specification is normative
            except sections explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
   <p> Examples in this specification are introduced with the words “for example”
            or are set apart from the normative text with <code>class="example"</code>, like this: </p>
   <div class="example" id="example-example"><a class="self-link" href="#example-example"></a> This is an example of an informative example. </div>
   <p> Informative notes begin with the word “Note”
            and are set apart from the normative text with <code>class="note"</code>, like this: </p>
   <p class="note" role="note"> Note, this is an informative note. </p>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#dictdef-elementinternals">ElementInternals</a><span>, in §1.2</span>
   <li><a href="#semantic-properties">semantic properties</a><span>, in §1.2</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-idl-def-accessibilityrole">
   <a href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole">https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-def-accessibilityrole">1.2. Changes to custom element definition</a> <a href="#ref-for-idl-def-accessibilityrole①">(2)</a> <a href="#ref-for-idl-def-accessibilityrole②">(3)</a> <a href="#ref-for-idl-def-accessibilityrole③">(4)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-def-ariaattributes">
   <a href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes">https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-def-ariaattributes">1.2. Changes to custom element definition</a> <a href="#ref-for-idl-def-ariaattributes①">(2)</a> <a href="#ref-for-idl-def-ariaattributes②">(3)</a> <a href="#ref-for-idl-def-ariaattributes③">(4)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-accessibility-tree">
   <a href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessibility-tree">https://www.w3.org/TR/core-aam-1.1/#dfn-accessibility-tree</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-accessibility-tree">1.1.1. Defining custom element semantics as part of the custom element definition</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-accessible-object">
   <a href="https://www.w3.org/TR/core-aam-1.1/#dfn-accessible-object">https://www.w3.org/TR/core-aam-1.1/#dfn-accessible-object</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-accessible-object">1.1.1. Defining custom element semantics as part of the custom element definition</a> <a href="#ref-for-dfn-accessible-object①">(2)</a>
    <li><a href="#ref-for-dfn-accessible-object②">1.1.2. Defining per-instance custom element semantics</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-mapping_general">
   <a href="https://www.w3.org/TR/core-aam-1.1/#mapping_general">https://www.w3.org/TR/core-aam-1.1/#mapping_general</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-mapping_general">1.1.1. Defining custom element semantics as part of the custom element definition</a>
    <li><a href="#ref-for-mapping_general①">1.1.2. Defining per-instance custom element semantics</a>
    <li><a href="#ref-for-mapping_general②">2.2.1. Rules for exposing semantics of custom elements</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-mapping_role">
   <a href="https://www.w3.org/TR/core-aam-1.1/#mapping_role">https://www.w3.org/TR/core-aam-1.1/#mapping_role</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-mapping_role">1.1.1. Defining custom element semantics as part of the custom element definition</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-element">
   <a href="https://dom.spec.whatwg.org/#element">https://dom.spec.whatwg.org/#element</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-element">2.1. Introduction</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-element-custom">
   <a href="https://dom.spec.whatwg.org/#concept-element-custom">https://dom.spec.whatwg.org/#concept-element-custom</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-element-custom">2.2.1. Rules for exposing semantics of custom elements</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-reflect">
   <a href="https://dom.spec.whatwg.org/#concept-reflect">https://dom.spec.whatwg.org/#concept-reflect</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-reflect">2.1. Introduction</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-customelementregistry">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry">https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-customelementregistry">1.2. Changes to custom element definition</a> <a href="#ref-for-customelementregistry①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-elementdefinitionoptions">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions">https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-elementdefinitionoptions">1.1.1. Defining custom element semantics as part of the custom element definition</a> <a href="#ref-for-elementdefinitionoptions①">(2)</a>
    <li><a href="#ref-for-elementdefinitionoptions②">1.2. Changes to custom element definition</a> <a href="#ref-for-elementdefinitionoptions③">(2)</a>
    <li><a href="#ref-for-elementdefinitionoptions④">2. ARIA semantic precedence between ElementDefinitionOptions, ElementInternals and ARIA properties</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-custom-element">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element">https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-custom-element">1. Defining custom element semantics</a>
    <li><a href="#ref-for-custom-element①">1.1.2. Defining per-instance custom element semantics</a>
    <li><a href="#ref-for-custom-element②">2.2.1. Rules for exposing semantics of custom elements</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-custom-element-definition">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition">https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-custom-element-definition">1.1. Introduction</a>
    <li><a href="#ref-for-custom-element-definition①">1.1.1. Defining custom element semantics as part of the custom element definition</a>
    <li><a href="#ref-for-custom-element-definition②">1.2. Changes to custom element definition</a> <a href="#ref-for-custom-element-definition③">(2)</a> <a href="#ref-for-custom-element-definition④">(3)</a>
    <li><a href="#ref-for-custom-element-definition⑤">2.1. Introduction</a>
    <li><a href="#ref-for-custom-element-definition⑥">2.2.1. Rules for exposing semantics of custom elements</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dom-customelementregistry-define">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define">https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-customelementregistry-define">1.1.1. Defining custom element semantics as part of the custom element definition</a>
    <li><a href="#ref-for-dom-customelementregistry-define①">1.2. Changes to custom element definition</a>
    <li><a href="#ref-for-dom-customelementregistry-define②">2.1. Introduction</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-custom-element-definition">
   <a href="https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition">https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-definition</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-custom-element-definition">1.1. Introduction</a>
    <li><a href="#ref-for-custom-element-definition①">1.1.1. Defining custom element semantics as part of the custom element definition</a>
    <li><a href="#ref-for-custom-element-definition②">1.2. Changes to custom element definition</a> <a href="#ref-for-custom-element-definition③">(2)</a> <a href="#ref-for-custom-element-definition④">(3)</a>
    <li><a href="#ref-for-custom-element-definition⑤">2.1. Introduction</a>
    <li><a href="#ref-for-custom-element-definition⑥">2.2.1. Rules for exposing semantics of custom elements</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-details-id-17">
   <a href="https://www.w3.org/TR/html-aam-1.0/#details-id-17">https://www.w3.org/TR/html-aam-1.0/#details-id-17</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-details-id-17">1.1.1. Defining custom element semantics as part of the custom element definition</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-ordered-map">
   <a href="https://infra.spec.whatwg.org/#ordered-map">https://infra.spec.whatwg.org/#ordered-map</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-ordered-map">1.1.1. Defining custom element semantics as part of the custom element definition</a>
    <li><a href="#ref-for-ordered-map①">1.1.2. Defining per-instance custom element semantics</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-DOMString">
   <a href="https://heycam.github.io/webidl/#idl-DOMString">https://heycam.github.io/webidl/#idl-DOMString</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-DOMString">1.2. Changes to custom element definition</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[aria]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-idl-def-accessibilityrole" style="color:initial">AccessibilityRole</span>
     <li><span class="dfn-paneled" id="term-for-idl-def-ariaattributes" style="color:initial">AriaAttributes</span>
    </ul>
   <li>
    <a data-link-type="biblio">[core-aam]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-dfn-accessibility-tree" style="color:initial">accessibility tree</span>
     <li><span class="dfn-paneled" id="term-for-dfn-accessible-object" style="color:initial">accessible object</span>
     <li><span class="dfn-paneled" id="term-for-mapping_general" style="color:initial">mapped</span>
     <li><span class="dfn-paneled" id="term-for-mapping_role" style="color:initial">mapped role</span>
    </ul>
   <li>
    <a data-link-type="biblio">[DOM]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-element" style="color:initial">Element</span>
     <li><span class="dfn-paneled" id="term-for-concept-element-custom" style="color:initial">custom</span>
     <li><span class="dfn-paneled" id="term-for-concept-reflect" style="color:initial">reflect</span>
    </ul>
   <li>
    <a data-link-type="biblio">[HTML]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-customelementregistry" style="color:initial">CustomElementRegistry</span>
     <li><span class="dfn-paneled" id="term-for-elementdefinitionoptions" style="color:initial">ElementDefinitionOptions</span>
     <li><span class="dfn-paneled" id="term-for-custom-element" style="color:initial">custom element</span>
     <li><span class="dfn-paneled" id="term-for-custom-element-definition" style="color:initial">custom element definition</span>
     <li><span class="dfn-paneled" id="term-for-dom-customelementregistry-define" style="color:initial">define(name, constructor)</span>
     <li><span class="dfn-paneled" id="term-for-custom-element-definition①" style="color:initial">defined</span>
    </ul>
   <li>
    <a data-link-type="biblio">[html-aam]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-details-id-17" style="color:initial">role of button</span>
    </ul>
   <li>
    <a data-link-type="biblio">[INFRA]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-ordered-map" style="color:initial">map</span>
    </ul>
   <li>
    <a data-link-type="biblio">[WebIDL]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-idl-DOMString" style="color:initial">DOMString</span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-dom">[DOM]
   <dd>Anne van Kesteren. <a href="https://dom.spec.whatwg.org/">DOM Standard</a>. Living Standard. URL: <a href="https://dom.spec.whatwg.org/">https://dom.spec.whatwg.org/</a>
   <dt id="biblio-html">[HTML]
   <dd>Anne van Kesteren; et al. <a href="https://html.spec.whatwg.org/multipage/">HTML Standard</a>. Living Standard. URL: <a href="https://html.spec.whatwg.org/multipage/">https://html.spec.whatwg.org/multipage/</a>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
   <dt id="biblio-webidl">[WebIDL]
   <dd>Boris Zbarsky. <a href="https://heycam.github.io/webidl/">Web IDL</a>. 15 December 2016. ED. URL: <a href="https://heycam.github.io/webidl/">https://heycam.github.io/webidl/</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-infra">[INFRA]
   <dd>Anne van Kesteren; Domenic Denicola. <a href="https://infra.spec.whatwg.org/">Infra Standard</a>. Living Standard. URL: <a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="idl-index"><span class="content">IDL Index</span><a class="self-link" href="#idl-index"></a></h2>
<pre class="idl highlight def">// Existing IDL
/*
[Exposed=Window]
interface CustomElementRegistry {
  [CEReactions] void define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions options);
  // ...
};

dictionary ElementDefinitionOptions {
  DOMString extends;
};
*/
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions②①"><c- n>ElementDefinitionOptions</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole①①"><c- n>AccessibilityRole</c-></a>;
<a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/custom-elements.html#elementdefinitionoptions" id="ref-for-elementdefinitionoptions③①"><c- n>ElementDefinitionOptions</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes①①"><c- n>AriaAttributes</c-></a>;

<c- b>dictionary</c-> <a href="#dictdef-elementinternals"><code><c- g>ElementInternals</c-></code></a> {};

<a class="n" data-link-type="idl-name" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals④①"><c- n>ElementInternals</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-accessibilityrole" id="ref-for-idl-def-accessibilityrole②①"><c- n>AccessibilityRole</c-></a>;
<a class="n" data-link-type="idl-name" href="#dictdef-elementinternals" id="ref-for-dictdef-elementinternals⑤①"><c- n>ElementInternals</c-></a> <c- b>includes</c-> <a class="n" data-link-type="idl-name" href="https://www.w3.org/TR/wai-aria-1.2/#idl-def-ariaattributes" id="ref-for-idl-def-ariaattributes②①"><c- n>AriaAttributes</c-></a>;

</pre>
  <aside class="dfn-panel" data-for="semantic-properties">
   <b><a href="#semantic-properties">#semantic-properties</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-semantic-properties">1.2. Changes to custom element definition</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dictdef-elementinternals">
   <b><a href="#dictdef-elementinternals">#dictdef-elementinternals</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dictdef-elementinternals">1.1.2. Defining per-instance custom element semantics</a> <a href="#ref-for-dictdef-elementinternals①">(2)</a> <a href="#ref-for-dictdef-elementinternals②">(3)</a> <a href="#ref-for-dictdef-elementinternals③">(4)</a>
    <li><a href="#ref-for-dictdef-elementinternals④">1.2. Changes to custom element definition</a> <a href="#ref-for-dictdef-elementinternals⑤">(2)</a>
    <li><a href="#ref-for-dictdef-elementinternals⑥">2. ARIA semantic precedence between ElementDefinitionOptions, ElementInternals and ARIA properties</a>
    <li><a href="#ref-for-dictdef-elementinternals⑦">2.1. Introduction</a> <a href="#ref-for-dictdef-elementinternals⑧">(2)</a> <a href="#ref-for-dictdef-elementinternals⑨">(3)</a> <a href="#ref-for-dictdef-elementinternals①⓪">(4)</a>
    <li><a href="#ref-for-dictdef-elementinternals①①">2.2.1. Rules for exposing semantics of custom elements</a> <a href="#ref-for-dictdef-elementinternals①②">(2)</a>
   </ul>
  </aside>
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